The present invention relates to a tandem type scanning optical system for an image formation device such as a color laser printer.
Tandem type scanning optical systems are typically employed in color image formation devices employing an electrophotographic imaging method. The tandem type scanning optical system is generally configured such that a plurality of beams emitted from a plurality of laser diodes are deflected by a single polygonal mirror to dynamically scan in a predetermined direction (which will be referred to as a main scanning direction), and the deflected beams are incident on a plurality of photoconductive drums for forming images of primary color components or complementary color components, respectively, via imaging optical systems (i.e., fθ lens). As exposed to scanning beams, latent images for the primary or complementary color components are formed on the photoconductive drums, respectively.
Examples of such a scanning optical system are disclosed in Japanese Patent Provisional Publications P2003-075751A (hereinafter, referred to as '751 publication) and P2003-149573A (hereinafter, referred to as '573 publication). According to these publications, the imaging optical system includes a first lens group arranged to receive all of the plurality of deflected beams, and a plurality of second lens groups that respectively receive the plurality of deflected beams. According to '751 publication and '573 publication, chromatic aberration of the second lens is not compensated for. Therefore, if the wavelengths of the plurality of laser beams emitted by the plurality of laser diodes are different, the chromatic aberration occurs. In such a case, due to the chromatic aberration, the length of the scanned lines (i.e., magnifications) of respective color components formed on the photoconductive drums may be different from each other.
Incidentally, a diffractive lens structure has been known as measures to compensate for the chromatic aberration due to the difference among wavelengths of the plurality of beams. Examples of such an application of the diffractive lens are disclosed in Japanese Patent Provisional Publications No. HEI 10-197820A (hereinafter, referred to as '820 publication) and P2001-142020A (hereinafter, referred to as '020 publication).
It should be noted, however, it is relatively difficult to form the diffractive lens structure in the scanning optical system disclosed in '751 publication and '573 publication.
When the imaging optical system consists of only the first and second optical systems, it is preferable to employ the diffractive lens structure on a surface of the first lens group since the first group of optical system is relatively small in size along the main scanning direction. On the other hand, since the diffractive lens structure, or the lens having the diffractive lens structure on a surface thereof is typically made of resin material and formed by injection molding, and it is necessary to form a molding die, it is preferable that the diffractive lens structure can be formed on a master block using a lathe. The diffractive lens structure for compensating for the lateral chromatic aberration has a pattern of steps repeated along the main scanning direction. That is, a boundary of adjoining diffraction areas extends in an auxiliary scanning direction (which is perpendicular to the main scanning direction). Therefore, a base curve on which the diffractive lens structure is formed is typically rotationally symmetrical about the optical axis or rotationally symmetrical about an axis extending in the main scanning direction (i.e., the base curve is arc-shaped cross section when cut along the auxiliary scanning direction).
If the rotationally symmetrical surface about the optical axis is formed in the first lens group, part of the beams incident on the first lens group after reflected by the polygonal mirror may be reflected by the surface rotationally symmetrical about the optical axis, reflected by an adjoining reflection surface of the polygonal mirror, incident on surfaces to be scanned via the imaging optical system as ghost light, and cause uneven density thickness.
Further, if the diffractive lens structure is formed on a lens surface, it is necessary to avoid deterioration of the diffractive lens structure due to contraction after molding. The diffractive lens structure for compensating for the longitudinal chromatic aberration has steps, whose side surfaces are parallel with the optical axis of the lens as well as the auxiliary scanning direction.
According to the optical system disclosed in '751 publication, the following problem arises. Only the surface of the lens whose shape in the auxiliary scanning direction is arc-shaped and thus, the mother die for which can be formed with the lathe is the target surface side surface of the elongated lens of the second lens group. However, it will take a relatively long period of time to form the mother die of the diffractive lens structure to be formed on this lens, which is elongated in the main scanning direction. Therefore, this elongated lens is not appropriate for such a purpose. It should be noted that, according to the '751 publication, the second lens of the first group is a glass lens, therefore, it is not appropriate for the lens on which the diffractive lens structure is formed.
According to the third embodiment of '573 publication, a polygonal mirror side surface of the scanning lens is a spherical surface, which is advantageous in forming the mother die thereof. However, since this surface is a concave surface, the shape of the diffractive lens may be broken due to contraction after molding. Therefore, this surface is inappropriate for the diffractive lens structure. The other embodiments of the '573 publication each consists of three groups of lenses, which may increase the manufacturing cost.